The equation of a line is y=2x+b. rewrite that equation in terms of b.

On both sides of the equal sign you subtract b (-b) which would give you -b+y=2x after that you subtract a y (-y) which gives you -b=2x-y now I was taught that you can't have a negative letter equal something so you change all of the signs to the opposite which would give you b=-2x+y